Crystalline aluminosilicate zeolite type molecular sieve are well known in the art and are formed by corner sharing SiO2 and AlO2 tetrahedra and have pore openings of uniform dimensions, have a significant ion exchange capacity and are capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without displacing any atoms which make up the permanent crystal structure.
The most recently synthesized molecular sieves without silica are the crystalline aluminophosphate compositions disclosed in the U.S. Pat. No. 4,310,440 (1982). These materials are formed from ALO2 and PO2 tetrahedra and have electroneutral frameworks as in the case of silica polymorphs. Unlike the silica molecular sieve, silicalite, which is hydrophobic due to the absence of extra framework cations, aluminophosphate molecular sieves are moderately hydrophilic apparently due to the difference in the electronegativities of aluminium and phosphorous. Their intracrystalline pore volumes and pore diameter are comparable are comparable to those known for zeolites and silica molecular sieves.
A novel class of silicon—substituted aluminophosphate molecular sieve is disclosed in Lok et al., (U.S. Pat. No. 4,440,871) in 1984 which are both crystalline and microporous and exhibit properties which are characteristic of both aluminosilicate zeolite and the aluminophosphates. Members of this novel class of silicoaluminophosphate materials have three dimensional crystal framework structure of PO2+AlO2− and SiO2 tetrahedral units, and whose essential empirical chemical composition on an anhydrous basis isMR:(Six Aly Pz)O2wherein ‘R’ represents at least one organic templating agent present in the intracrystalline pore system, ‘m’ represents the moles of ‘R’ present per mole of (Six Aly Pz)O2 and has a value of from 0.1 to 0.3 and x, y, and z have a value from 0.1 to 0.20, 0.4 to 0.59 and 0.40 to 0.59 respectively.
Aluminophosphates are neutral in that they do not processes any acidity as the (AlO2)− and (PO2)+ tetrahedra alternate in a regular fashion in the structure leading to a neutral framework. However, when (SiO2) tetrahedra are introduced in the lattice, they can replace a (PO2)+ tetrahedra leading to an anionic (negative) charge on the framework which when neutralized by a H+ (proton) leads to an acid site just at the replacement of Si4+ ions by Al3+ and an P5+ ions simultaneously replaced by two Si4+ ions, no acidity is generated due to exact charge balancing.
It has been found that the method of preparation of the SAPO molecular sieve can influence the manner of incorporation of Si4+ ions and the acidity of the material. It has been reported that the synthesis of SAPO-35 in a non-aqueous medium produces a more acidic material through preferential replacement of P5+ ions by Si5+ ions than when synthesis is carried out in an aqueous medium (Venkatathari and others in J. C. S. Faraday Transactions, volume 93, page 3411, year 1997).
The prior art procedure for the synthesis of SAPO-35 as disclosed in the U.S. Pat. No. 4,440,871 involves the reaction of aluminium isopropoxide or hydrated aluminum oxide with phosphoric acid and silica sol or fumed silica in the presence of organic templating compounds such as cyclohexyl amine and quinuclidine by autoclaving the reaction mixture directly at an elevated temperature of 150° C. to 200° C. for 24 h to 168 h.
The major disadvantages of the above prior art method for the synthesis of SAPO-35 is lower Bronsted acidity due to lower incorporation of Si in the framework besides producing less crystalline sample.